[subset] when no --output-file=, use INPUT_FILE.subset.{extension}

We should probably use cliTools.makeOutputFileName here for consistency with ttx,
but I know some prefer the approach taken by pyftsubset so I don't want force this.

Howver, I find it a bit annoying that when one is too lazy (like me) to specify
the --output-file=, the subsetter outputs a file ending with an invalid ".subset" extension, which Finder or Windows Explorer don't recongize as a font, and so one has to rename it anyway.

This makes the '.subset' string is inserted between the file name and its original extension (if any).

Lib/fontTools/subset/__init__.py

@@ -2866,6 +2866,7 @@ def usage():
 
 @timer("make one with everything (TOTAL TIME)")
 def main(args=None):
+    from os.path import splitext
     from fontTools import configLogger
 
     if args is None:
@@ -2902,7 +2903,8 @@ def main(args=None):
     args = args[1:]
 
     subsetter = Subsetter(options=options)
-    outfile = fontfile + '.subset'
+    basename, extension = splitext(fontfile)
+    outfile = basename + '.subset' + extension
     glyphs = []
     gids = []
     unicodes = []